Light emitting element and light emitting device

ABSTRACT

A light emitting element of the invention includes n pieces of light emitting layers (n is a natural number) between first and second electrodes. A first layer and a second layer are provided between the m th  light emitting layer (m is a natural number of 1≦m≦n) and the m+1 th  light emitting layer. The first and second layers are contacted to each other. The first layer contains a substance that transports holes easily and a substance with an electron accepting property. The second layer contains a substance that transports electrons easily and a substance with an electron donating property. Molybdenum oxide is used as the substance with the electron accepting property.

This application is a continuation of application Ser. No. 13/101,572filed on May 5, 2011 which is a continuation of application Ser. No.12/563,242 filed on Sep. 21, 2009 (now U.S. Pat. No. 7,940,002 issuedMay 10, 2011) which is a continuation of application Ser. No. 10/593,193filed on Sep. 15, 2006 (now U.S. Pat. No. 7,598,670 issued Oct. 6, 2009)which claims priority under 35 USC 371 of PCT/JP2005/009313 filed May17, 2005.

TECHNICAL FIELD

The present invention relates to a light emitting element having a lightemitting layer between a pair of electrodes, and in particular, relatesto a layer structure of the light emitting element.

BACKGROUND ART

A light emitting device utilizing light emitted from anelectroluminescent element (a light emitting element) has wide-viewingangle and low power consumption. In recent years, research anddevelopment of a light emitting device, which can provide high qualityimages for the long term, have been carried out actively in thedevelopment area of light emitting devices so as to dominate the marketfor display devices that are applied to various kinds of informationprocessing devices such as a television receiver and a car navigationsystem.

In order to obtain a light emitting device, which can provide highquality images for the long term, development of a long-life lightemitting element and a light emitting element that emits lightefficiently becomes important.

For example, the patent document 1 discloses a technique related to alight emitting element with a plurality of light emitting units in whichthe respective light emitting units are separated by a charge generatinglayer. The patent document 1 describes a long-life light emittingelement with high luminance. However, vanadium pentoxide used in thepatent document 1 has a high moisture absorbing property. Therefore, thelight emitting element is possibly deteriorated due to moisture absorbedby vanadium pentoxide. The deterioration of the light emitting elementresults in deterioration of image quality in a light emitting device.

Accordingly, in the development of light emitting devices, it is alsoimportant to manufacture a light emitting element having a high moistureresistant property along with high luminance.

-   [Patent Document 1] Japanese Patent Application Laid-Open No.    2003-272860.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a light emittingelement with an excellent moisture resistant property. In particular, itis an object of the invention to provide a light emitting element withan excellent moisture resistant property that can emit white light.

In an aspect of the invention, a light emitting element includes a layercontaining a substance with an electron accepting property. As thesubstance with the electron accepting property, molybdenum oxide isused.

In another aspect of the invention, a light emitting element includes npieces of light emitting layers (n is a natural number) between a firstelectrode and a second electrode. The light emitting element furtherincludes a first layer and a second layer between the m^(th) lightemitting layer (m is a natural number: 1≦m≦n) and the m+1^(th) lightemitting layer. The first and second layers are contacted to each other.The first layer contains a substance that transports holes easily and asubstance with an electron accepting property. The second layer containsa substance that transports electrons easily and a substance with anelectron donating property. In addition, molybdenum oxide is used as thesubstance with the electron donating property.

In another aspect of the invention, a light emitting element includes npieces of layer groups (n is a natural number) each of which has a firstlayer, a second layer and a light emitting layer, between a pair ofelectrodes. The first layer includes a substance that transports holeseasily and a substance with an electron accepting property. The secondlayer includes a substance that transports electrons easily and asubstance with an electron donating property. In the n pieces of layergroups, the first layer included in the m^(th) layer group (m is anatural number: 1≦m≦n) and the second layer included in the m+1^(th)layer group are laminated to be in contact with each other.

In another aspect of the invention, a light emitting element includes npieces of light emitting layers (n is a natural number) between a firstelectrode and a second electrode. The second electrode reflects lightmore easily as compared with the first electrode. The light emittingelement further includes a first layer and a second layer between them^(th) light emitting layer (m is a natural number: 1=m=n) and them+1^(th) light emitting layer. Further, the first and second layers arecontacted to each other. The first layer includes a substance thattransports holes easily and a layer with an electron accepting property.The second layer includes a substance that transports electrons easilyand a substance with an electron donating property. The m+1^(th) lightemitting layer exhibits a shorter peak wavelength of emission spectrumthan that of the m^(th) light emitting layer. The n pieces of lightemitting layers are provided such that the m+1^(th) light emitting layeris placed closer to the second electrode than the m^(th) light emittinglayer.

In another aspect of the invention, a light emitting element includes npieces of light emitting layers (n is a natural number) between a firstelectrode and a second electrode. The second electrode reflects lightmore easily as compared with the first electrode. The light emittingelement further includes a first layer and a second layer between the mmlight emitting layer (m is a natural number 1=m=n) and the m+1^(th)light emitting layer. The first and second layers are contacted to eachother. The first layer contains a substance that transports holes easilyand a substance with an electron accepting property. The second layercontains a substance that transports electrons easily and a substancewith an electron donating property. The n pieces of light emittinglayers are provided such that the light emitting layer exhibiting ashorter peak wavelength of emission spectrum is provided closer to thesecond electrode.

According to the present invention, a light emitting element with anexcellent moisture resistant property can be obtained so that the lightemitting element is hardly deteriorated by moisture intruding into thelight emitting element. Also, a light emitting element that emits whitelight can be provided. In addition, since an interference of lightemitted from the light emitting element and reflected light is hardlycaused in the light emitting element of the invention, color tone oflight emitted from the light emitting element can be controlled easily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram explaining a light emitting element of the presentinvention;

FIG. 2 is a diagram explaining a light emitting element of theinvention;

FIG. 3 is a top view showing a light emitting device according to theinvention;

FIG. 4 is a diagram explaining a circuit included in a light emittingdevice according to the invention;

FIG. 5 is a top view of a light emitting device according to theinvention;

FIG. 6 is a diagram explaining a frame operation of a light emittingdevice according to the invention;

FIGS. 7A to 7C are cross sectional views of light emitting devicesaccording to the invention;

FIGS. 8A to 8C are diagrams showing electronic appliances according tothe invention;

FIG. 9 is a diagram explaining a light emitting element according to theinvention;

FIG. 10 is a graph showing emission spectrum of a light emitting elementaccording to the invention; and

FIG. 11 is a graph showing transmission spectrum with respect to a mixedlayer of molybdenum oxide and α-NPD.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment modes and embodiments according to the present inventionwill hereinafter be described referring to the accompanying drawings andthe like. Further, the present invention can be carried out in manydifferent modes. It is easily understood by those who skilled in the artthat the embodiment modes and details herein disclosed can be modifiedin various ways without departing from the purpose and the scope of theinvention. The present invention should not be interpreted as beinglimited to the description of the embodiment modes to be given below.

[Embodiment Mode 1]

In FIG. 1, a first layer 103 containing a substance that transportsholes easily and a substance with an electron accepting property and asecond layer 104 containing a substance that transports electrons easilyand a substance with an electron donating property are provided betweena first electrode 101 and a second electrode 102. The first layer 103and the second layer 104 are laminated in contact with each other.Further holes are generated in the first layer 103 containing thesubstance that transports holes easily and the substance with theelectron accepting property whereas electrons are generated in thesecond layer 104 containing the substance that transports electronseasily and the substance with the electron donating property.

Also, a first light emitting layer 111 is provided between the firstelectrode 101 and the first layer 103. A second light emitting layer 121is provided between the second electrode 102 and the second layer 104.

Further, in this embodiment mode, the first electrode 101 serves as ananode and the second electrode 102 serves as a cathode. Preferably, oneor both of the first electrode 101 and the second electrode 102 is/aremade from a substance that transmits visible light easily.

A substance for the first electrode 101 is not particularly limited. Inorder to form the first electrode 101 serving as the anode as well asthis embodiment mode, the first electrode is preferably made from asubstance with a high work function such as indium tin oxide, indium tinoxide containing silicon oxide, indium zinc oxide in which 2 to 20% zincoxide is mixed in indium oxide, and gallium zinc oxide in which several% gallium oxide is mixed in zinc oxide. Further, the electrode made fromthe above-mentioned substance with the high work function transmitsvisible light easily.

In addition, a substance for the second electrode 102 is notparticularly limited. In order to form the second electrode 102 servingas the cathode as well as this embodiment mode, the second electrode ispreferably made from a substance with a low work function such asaluminum containing alkali metal (e.g., lithium (a), magnesium and thelike), alkali earth metal or the like.

The substance that transports holes easily is not particularly limited.For example, the substance that transports holes easily can be made froman aromatic amine (i.e., one having a benzene ring-nitrogen bond)compound such as: 4,4′-bis(N-[1-naphthyl]-N-phenyl-amino)-biphenyl(abbreviation: α-NPD);4,4′-bis(N-[3-methylphenyl]-N-phenyl-amino)-biphenyl (abbreviation:TPD); 4,4′,4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviation:TDATA); and4,4′,4″-tris(N-(3-methylphenyl)-N-phenyl-amino)-triphenylamine(abbreviation: MTDATA).

The substance with the electron accepting property is not particularlylimited. For example, a substance with a low moisture absorbing propertysuch as molybdenum oxide is preferably employed.

The substance that transports electrons easily is not particularlylimited. For example, the substance that transports electrons easily canbe made from a metal complex having quinoline skeleton or benzoquinolineskeleton such as: tris(8-quinolinolate)aluminum (abbreviation: Alq₃);tris(4-methyl-8-quinolinolate)aluminum (abbreviation: Almq₃);bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂); andbis(2-methyl-8-quinolinolate)-4-pheylphenolate-aluminum (abbreviation:BAlq).

The substance with the electron donating property is not particularlylimited. For example, alkali metal such as lithium, alkali earth metalsuch as magnesium and the like can be used. In addition, alkali metaloxide such as lithium oxide, alkali metal nitride such as lithiumnitride, alkali earth metal oxide such as magnesium oxide, alkali earthmetal nitride such as magnesium nitride may be employed.

The first light emitting layer 111 and the second light emitting layer121 contain a light emitting substance, respectively. The light emittingsubstance mentioned above indicates a substance with a favorable lightemitting efficiency exhibiting light emission of a predeterminedwavelength. Further, the first and second light emitting layers 111 and121 may contain different light emitting substances from each other. Thefirst and second light emitting layers 111 and 121 are not particularlylimited. Each layer may be made from a layer containing one kind ofsubstance or a layer in which plural kinds of substances are mixed. Forinstance, any one or both of the first and second light emitting layers111 and 121 may be made from only a light emitting substance.Alternatively, any one or both of the first and second light emittinglayers may be formed of a mixed layer of a light emitting substance andother substance. As the substance used in combination with the lightemitting substance, a substance having a larger energy gap than that ofthe light emitting substance is preferably used. In this embodimentmode, the energy gap indicates an energy gap between the LUMO level andthe HOMO level.

The light emitting substance is not particularly limited. In order toobtain red light emission, for example, the following substancesexhibiting emission spectrum with peaks of at 600 nm to 680 nm can beemployed: 4-dicyanomethylene-2-isoproopyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran (abbreviation:DCJTI);4-dicyanomethylene-2-methyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran(abbreviation: DCTJ);4-dicyanomethylene-2-tert-butyl-6-(2-[1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)-4H-pyran;periflanthene;2,5-dicyano-1,4-bis(2-[10-methoxy-1,1,7,7-tetramethyljulolidine-9-yl]ethenyl)benzeneand the like. In order to obtain green light emission, substancesexhibiting emission spectrum with peaks at 500 nm to 550 nm such asN,N′-dimethyl-quinacridon (abbreviation: DMQd), coumarin 6, coumarin545T, and tris(8-quinolinolate)aluminum (abbreviation Alq₃) can beemployed. In order to obtain blue light emission, the followingsubstances exhibiting emission spectrum with peaks at 420 nm to 480 nmcan be employed: 9,10-bis(2-naphthyl)-tert-butylanthracene(abbreviation: t-BuDNA); 9, 9′-bianthryl; 9,10-diphenylanthracene(abbreviation: DPA); 9,10-bis(2-naphthyl)anthracene (abbreviation: DNA);bis(2-methyl-8-quinolinolate)-4-phenylphenolate-gallium (abbreviation:BGaq); bis(2-methyl-8-quinolinolate)-4-phenylphenolate-aluminum(abbreviation: BAlq); and the like.

In the above-mentioned light emitting elements, when a voltage isapplied to the first and second electrodes 101 and 102, holes areinjected in the first light emitting layer 111 from the first electrode101, and electrons are injected in the first light emitting layer 111from the first layer 103. Also, holes are injected in the second lightemitting layer 121 from the second electrode 102. Electrons are injectedin the second light emitting layer 121 from the second layer 104.Accordingly, the holes and electrons are recombined in the first andsecond light emitting layers 111 and 121 so that the light emittingsubstance is excited. The light emitting substance emits light uponreturning to a ground state from the excitation state.

Further, when a color of light emitted from the first emitting layer 111and a color of light emitted from the second light emitting layer 121are complementary to each other, the human eye detects the light emittedfrom both the light emitting elements as white. In this case, when thereflectance of the first electrode 101 is different from that of thesecond electrode 102 and the peak wavelengths of emission spectrum oflight emitted from the respective light emitting layers (i.e.,wavelengths of maximum emission intensity in the case of inspecting theemission spectrums) are different from each other, the respective lightemitting layers are preferably arranged such that the light emittinglayer with a shorter peak wavelength is placed closer to the electrodewith the high reflectance. Further, the peak wavelength indicates awavelength exhibiting a peak with a strongest emission intensity in theemission spectrum having a plurality of peaks. For example, in the casewhere the first electrode 101 is made from indium tin oxide or the likeand transmits visible light easily while the second electrode 102 ismade from aluminum or the like and reflects light easily, the firstlight emitting layer 111, which is placed closest to the first electrode101, is preferably a light emitting layer that emits blue light and thesecond light emitting layer 121, which is placed closest to the secondelectrode 102, is preferably a light emitting layer that emits yellowlight. Therefore, the light interference caused by reflecting lightemitted from the light emitting layers at the second electrode 102 canbe reduced.

Furthermore, only the first light emitting layer 111 may be providedbetween the first electrode 101 and the first layer 103 in the samemanner as this embodiment mode. Alternatively, a hole transporting layerand the like may be provided therebetween, besides the first lightemitting layer 111. Meanwhile, only the second light emitting layer 121may be provided between the second electrode 102 and the second layer104 as shown in this embodiment mode. Alternatively, an electrontransporting layer and the like may be provided therebetween, besidesthe second light emitting layer 121.

Since the above-described light emitting element according to theinvention is formed by using a substance with a low water absorbingproperty such as molybdenum oxide, the light emitting element is hardlydeteriorated by moisture intruding into the light emitting element. Inaddition, the light emitting element of this embodiment mode can emitwhite light. Moreover, the interference of light emitted from the lightemitting layers and reflected light is hardly caused in the lightemitting element of this embodiment mode, and hence, color tone of lightemitted from the light emitting layers can be controlled easily.

[Embodiment Mode 2]

The present embodiment mode will describe a light emitting element ofthe invention including three light emitting layers with reference toFIG. 2.

In FIG. 2, first layers 203, 205 and 207 including a substance thattransports electrons easily and a substance with an electron donatingproperty, and second layers 204, 206 and 208 including a substance thattransports holes easily and a substance with an electron acceptingproperty are provided between a first electrode 201 and a secondelectrode 202. In this case, the first layer 203 is formed in contactwith the first electrode 201. The second layer 208 is formed in contactwith the second electrode 202. The first layer 205 and the second layer204 are laminated to be in contact with each other. The first layer 207and the second layer 206 are laminated to be in contact with each other.Holes are generated in the first layers 203, 205 and 207 including thesubstance that transports holes easily and the substance with theelectron accepting property whereas electrons are generated in thesecond layers 204, 206 and 208 including the substance that transportselectrons easily and the substance with the electron donating property.

In this embodiment mode, the first electrode 201 serves as an anode andthe second electrode 202 serves as a cathode. A first light emittinglayer 211 is provided between the first layer 203 and the second layer204. A second light emitting layer 221 is provided between the firstlayer 205 and the second layer 206. A third light emitting layer 231 isprovided between the first layer 207 and the second layer 208. A holetransporting layer 212 is provided between the second layer 204 and thefirst light emitting layer 211. A hole transporting layer 222 isprovided between the second layer 206 and the second light emittinglayer 221. A hole transporting layer 232 is provided between the secondlayer 208 and the third light emitting layer 231. In this case, the holetransporting layers represent layers that can transport holes to thelight emitting layers and contain a substance, which transport holeseasily. An electron transporting layer 213 is provided between the firstlayer 203 and the first light emitting layer 211. An electrontransporting layer 223 is provided between the first layer 205 and thesecond light emitting layer 221. An electron transporting layer 233 isprovided between the first layer 207 and the third light emitting layer231. These electron transporting layers represent layers that cantransport electrons to the light emitting layers and contain asubstance, which transports electrons easily. By providing these holetransporting layers and these electron transporting layers, the lightemitting layers can be separated from the layers containing metal,thereby preventing light quenching caused by metal.

The substance that transports holes easily, the substance with theelectron accepting property, the substance that transports electronseasily, and the substance with the electron donating property areidentical to those described in Embodiment Mode 1. The substancesdescribed in Embodiment Mode 1 can be used. Further, as the substancewith the electron accepting property, a substance with a low moistureabsorbing property such as molybdenum oxide is preferably used in thisembodiment mode.

The first electrode 201 serves as the anode and the second electrode 202serves as the cathode. Therefore, the first electrode 201 is preferablymade from a substance with a high work function as well as the firstelectrode 101 of Embodiment Mode 1. Also, the second electrode 202 ispreferably made form a substance with a low work function as well as thesecond electrode 102 of Embodiment Mode 1. Preferably, one or both ofthe first electrode 201 and the second electrode 202 is/are made from asubstance that transmits visible light easily.

The hole transporting layers 212, 222 and 232 correspond to layerscontaining a substance that transports holes easily, respectively. Thehole transporting layers 212, 222 and 232 are not particularly limited.These hole transporting layers may contain different kinds of substancesthat transport holes easily or the same substance that transports holeseasily. In addition, the hole transporting layers 212, 222 and 232 maycontain one or more kinds of substances that transport holes easily,respectively. The hole transporting layers 212, 222 and 232 may includea single layer or plural layers, respectively. Further, the substancesthat transport holes easily are identical to the substance thattransports holes easily as mentioned in Embodiment Mode 1.

The electron transporting layers 213, 223 and 233 contain a substancethat transports electrons easily, respectively. The electrontransporting layers 213, 223 and 233 are not particularly limited. Theelectron transporting layers may contain different kinds of substancesthat transport electrons easily or the same substance that transportselectrons easily. The electron transporting layers 213, 223 and 233 maycontain one or more kinds of substances that transport electrons easily,respectively. The electron transporting layers 213, 223 and 233 mayinclude one layer or plural layers, respectively. Further, thesubstances that transport electrons easily are identical to thesubstance that transports electrons easily as described in EmbodimentMode 1.

The first light emitting layer 211, the second light emitting layer 221and the third light emitting layer 231 contain light emittingsubstances, respectively. The light emitting substances are identical tothe light emitting substance described in Embodiment Mode 1, andtherefore, the light emitting substance of Embodiment Mode 1 can beemployed here. In addition, the light emitting substances included inthe first light emitting layer 211, the second light emitting layer 221and the third light emitting layer 231 may be different from oneanother. Further, the first light emitting layer 211, the second lightemitting layer 221 and the third light emitting layer 231 are notparticularly limited. Each light emitting layer may include one kind ofsubstance or a mixture of different kinds of substances. For instance,any one or more of the first light emitting layer 211, the second lightemitting layer 221 and the third light emitting layer 231 may be madefrom only a light emitting substance. Alternatively, any one or more ofthe light emitting layers may be made from a mixture of a light emittingsubstance and another substance. As the substance used in combinationwith the light emitting substance, a substance having a larger energygap than that of the light emitting substance is preferably used. Inthis case, the energy gap indicates an energy gap between the LUMO leveland the HOMO level.

When any one of the first light emitting layer 211, the second lightemitting layer 221 and the third light emitting layer 231 emits redlight, another one emits green light and another one emits blue light,the human eye detects light emitted from the light emitting element aswhite. In the case where the light reflectance of the first electrode201 is different from that of the second electrode 202 and the peakwavelengths of emission spectrum of the respective light emitting layersare different from each other, the respective light emitting layers arepreferably arranged such that the light emitting layer with a shorterpeak wavelength is placed closer to the electrode having the highreflectance. Further, the peak wavelength indicates a wavelengthexhibiting a peak with a strongest emission intensity in the emissionspectrum having plural peaks. For example, when the first electrode 201is made from indium tin oxide or the like and transmits visible lighteasily while the second electrode 202 is made from aluminum or the likeand reflects light easily, the first light emitting layer 211 placedclosest to the first electrode 201 preferably corresponds to a lightemitting layer that emits red light and the third light emitting layer231 placed closest to the second electrode 202 preferably corresponds toa light emitting layer that emits blue light. Accordingly, lightinterference caused by reflecting light emitted from the light emittinglayers at the second electrode 202 can be reduced.

Since the above-mentioned light emitting element is formed by using thesubstance with the low moisture absorbing property such as molybdenumoxide, the light emitting element is hardly deteriorated by moistureintruding into the light emitting element. In addition, the lightemitting element of this embodiment mode can emit white light. Moreover,interference of light emitted from the light emitting layers andreflected light is hardly caused in the light emitting element of thepresent embodiment mode, and hence, color tone of light emitted from thelight emitting layers can be controlled easily.

[Embodiment Mode 3]

The light emitting element of the invention as described in EmbodimentMode 1 or 2 can be applied to a pixel portion of a light emitting devicehaving a display function or a lighting portion of a light emittingdevice having a lighting function.

The present embodiment mode will describe a circuit configuration of alight emitting device having a display function and a method for drivingthereof with reference to FIGS. 3 to 6.

FIG. 3 is a schematic view seen from a top face of the light emittingdevice according to the invention. In FIG. 3, a pixel portion 6511, asource signal line driver circuit 6512, a writing gate signal linedriver circuit 6513 and an erasing gate signal line driver circuit 6514are provided over a substrate 6500. The source signal line drivercircuit 6512, the writing gate signal line driver circuit 6513 and theerasing gate signal line driver circuit 6514 are connected to FPCs(flexible printed circuits) 6503 that are external input terminalsthrough wiring groups, respectively. The source signal line drivercircuit 6512, the writing gate signal line driver circuit 6513 and theerasing gate signal line driver circuit 6514 respectively receive videosignals, clock signals, start signals, reset signals and the like fromthe FPCs 6503. The FPCs 6503 are attached with a printed wiring board(PWB) 6504. Further, a driver circuit portion is not necessary to beprovided over the same substrate as the pixel portion 6511 as mentionedabove. For example, the driver circuit portion may be provided outsideof the substrate by utilizing an FPC with a wiring pattern over which anIC chip is mounted (TCP), or the like.

In the pixel portion 6511, a plurality of source signal liens extendingin columns are aligned in rows. Current supply lines are aligned inrows. Also, a plurality of gate signal lines extending in rows arealigned in columns in the pixel portion 6511. Additionally, a pluralityof circuits including light emitting elements are aligned in the pixelportion 6511.

FIG. 4 is a diagram showing a circuit for activating one pixel. Thecircuit as shown in FIG. 4 comprises a first transistor 901, a secondtransistor 902 and a light emitting element 903.

Each of the first and second transistors 901 and 902 is a three terminalelement including a gate electrode, a drain region and a source region.A channel region is provided between the drain region and the sourceregion. The region serving as the source region and the region servingas the drain region are changed depending on a configuration of atransistor an operational condition and the like, and therefore, it isdifficult to determine which regions serve as the source region and thedrain region. Accordingly, the regions serving as the source and thedrain are denoted as a first electrode and a second electrode in thisembodiment mode, respectively.

A gate signal line 911 and a writing gate signal line driver circuit 913are provided to be electrically connected or disconnected to each otherby a switch 918. The gate signal line 911 and an erasing gate signalline driver circuit 914 are provided to be electrically connected ordisconnected to each other by a switch 919. A source signal line 912 isprovided to be electrically connected to either a source signal linedriver circuit 915 or a power source 916 by a switch 920. A gate of thefirst transistor 901 is electrically connected to the gate signal line911. The first electrode of the first transistor is electricallyconnected to the source signal line 912 while the second electrodethereof is electrically connected to a gate electrode of the secondtransistor 902. The first electrode of the second transistor 902 iselectrically connected to a current supply line 917 while the secondelectrode thereof is electrically connected to one electrode included ina light emitting element 903. Further, the switch 918 may be included inthe writing gate signal line driver circuit 913. The switch 919 may alsobe included in the erasing gate signal line driver circuit 914. Inaddition, the switch 920 may be included in the source signal linedriver circuit 915.

The arrangement of the transistors, the light emitting element and thelike in the pixel portion is not particularly limited. For example, thearrangement as shown in a top view of FIG. 5 can be employed. In FIG. 5,a first electrode of a first transistor 1001 is connected to a sourcesignal line 1004 while a second electrode of the first transistor isconnected to a gate electrode of a second transistor 1002. A firstelectrode of the second transistor is connected to a current supply line1005 and a second electrode of the second transistor is connected to anelectrode 1006 of a light emitting element. A part of the gate signalline 1003 functions as a gate electrode of the first transistor 1001.

Next, the method for driving the light emitting device will bedescribed. FIG. 6 is a diagram explaining an operation of a frame withtime. In FIG. 6, a horizontal direction indicates time passage while alongitudinal direction indicates the number of scanning stages of a gatesignal line.

When an image is displayed on the light emitting device of theinvention, a rewriting operation and a displaying operation are carriedout repeatedly. The number of rewriting operations is not particularlylimited. However, the rewriting operation is preferably performed about60 times a second such that a person who watches a displayed image doesnot detect flicker in the image. A period of operating the rewritingoperation and the displaying operation of one image (one frame) is,herein, referred to as one frame period.

As shown in FIG. 6, one frame is divided into four sub-frames 501, 502,503 and 504 including writing periods 501 a, 502 a, 503 a and 504 a andholding periods 501 b, 502 b, 503 b and 504 b. The light emittingelement applied with a signal for emitting light emits light during theholding periods. The length ratio of the holding periods in each of thefirst sub-frame 501, the second sub-frame 502, the third sub-frame 503and the fourth sub-frame 504 satisfies 2³:2²:2¹:2⁰=8:4:2:1. This allowsthe light emitting device to exhibit 4-bit gray scale. Further, thenumber of bits and the number of gray scales are not limited to those asshown in this embodiment mode. For instance, one frame may be dividedinto eight sub-frames so as to achieve 8-bit gray scale.

The operation in one frame will be described. In the sub-frame 501, thewriting operation is first performed in 1^(st) row to a last row,sequentially. Therefore, the starting time of the writing periods isvaried for each row. The holding period 501 b sequentially starts in therows in which the writing period 501 a has been terminated. In theholding period 501 b, a light emitting element applied with a signal foremitting light remains in a light emitting state. Upon terminating theholding period 501 b, the sub-frame 501 is changed to the next sub-frame502 sequentially in the rows. In the sub-frame 502, a writing operationis sequentially performed in the 1^(st) row to the last row in the samemanner as the sub-frame 501. The above-mentioned operations are carriedout repeatedly up to the holding period 504 b and then terminated. Afterterminating the operation in the sub-frame 504, an operation in the nextframe is started. Accordingly, the sum of the light-emitting time inrespective sub-frames corresponds to the light emitting time of eachlight emitting element in one frame. By changing the light emitting timefor each light emitting element and combining such the light emittingelements variously within one pixel, various display colors withdifferent brightness and different chromaticity can be formed.

When the holding period is intended to be forcibly terminated in the rowin which the writing period has already been terminated and the holdingperiod has started prior to terminating the writing operation up to thelast row as shown in the sub-frame 504, an erasing period 504 c ispreferably provided after the holding period 504 b so as to stop lightemission forcibly. The row where light emission is forcibly stopped doesnot emit light for a certain period (this period is referred to as a nonlight emitting period 504 d). Upon terminating the writing period in thelast row, a writing period of a next sub-frame (or, a next frame) startssequentially from a first row. This can prevent the writing period inthe sub-frame 504 from overlapping with the writing period in the nextsub-frame.

Although the sub-frames 501 to 504 are arranged in order of increasingthe length of the holding period in this embodiment mode, they are notnecessary to be arranged in this order. For example, the sub-frames maybe arranged in ascending order of the length of the holding period.Alternatively, the sub-frames may be arranged in random order. Inaddition, these sub-frames may further be divided into a plurality offrames. That is, scanning of gate signal lines may be performed atseveral times during a period of supplying same video signals.

The operations in the wiring period and the erasing period of thecircuits as shown in FIG. 4 will be described.

The operation in the writing period will be described first. In thewriting period, the gate signal line 911 in the x^(th) row (x is anatural number) is electrically connected to the writing gate signalline driver circuit 913 via the switch 918. The gate signal line 911 inthe x^(th) row is not connected to the erasing gate signal line drivercircuit 914. The source signal line 912 is electrically connected to thesource signal line driver circuit 915 via the switch 920. In this case,a signal is input in a gate of the first transistor 901 connected to thegate signal line 911 in the x^(th) row (x is a natural number), therebyturning the first transistor 901 on. At this moment, video signals aresimultaneously input in the source signal lines in the first to lastcolumns. Further, the video signals input from the source signal line912 in each column are independent from one another. The video signalsinput from the source signal line 912 are input in a gate electrode ofthe second transistor 902 via the first transistor 901 connected to therespective source signal lines. At this moment, it is decided whetherthe light emitting element 903 emits light or emits no light dependingon a signal input in the second transistor 902. For instance, when thesecond transistor 902 is a p-channel type, the light emitting element903 emits light by inputting a low level signal in the gate electrode ofthe second transistor 902. On the other hand, when the second transistor902 is an n-channel type, the light emitting element 903 emits light byinputting a high level signal in the gate electrode of the secondtransistor 902.

Next, the operation in the erasing period will be described. In theerasing period, the gate signal line 911 in the x^(th) row (x is anatural number) is electrically connected to the erasing gate signalline driver circuit 914 via the switch 919. The gate signal line 911 inthe x^(th) row is not connected to the writing gate signal line derivercircuit 913. The source signal line 912 is electrically connected to thepower source 916 via the switch 920. In this case, upon inputting asignal in the gate of the first transistor 901 connected to the gatesignal line 911 in the x^(th) row, the first transistor 901 is turnedon. At this moment, erasing signals are simultaneously input in thesource signal lines in the first to last columns. The erasing signalsinput from the source signal line 912 are input in the gate electrode ofthe second transistor 902 via the first transistor 901 connected to therespective source signal lines. A supply of current flowing through thelight emitting element 903 from the current supply line 917 is forciblystopped by the signals input in the second transistor 902. This makesthe light emitting element 903 emit no light forcibly. For example, whenthe second transistor 902 is a p-channel type, the light emittingelement 903 emits no light by inputting a high level signal in the gateelectrode of the second transistor 902. On the other hand, when thesecond transistor 902 is an n-channel type, the light emitting element903 emits no light by inputting a low level signal in the gate electrodeof the second transistor 902.

Further, in the erasing period, a signal for erasing is input in thex^(th) row (x is a natural number) by the above-mentioned operation.However, as mentioned above, the x^(th) row sometimes remains in theerasing period while another row (e.g., a y^(th) row (y is a naturalnumber)) remains in the writing period. In this case, since a signal forerasing is necessary to be input in the x^(th) row and a signal forwriting is necessary to be input in the y^(th) row by utilizing thesource signal lines in the same columns, the after-mentioned operationis preferably carried out.

After the light emitting element 903 in the x^(th) row becomes anon-light emitting state by the above-described operation in the erasingperiod, the gate signal line and the erasing gate signal line drivercircuit 914 are immediately disconnected to each other and the sourcesignal line is connected to the source signal line driver circuit 915 byturning the switch 920 on. The gate signal line and the writing gatesignal line driver circuit 913 are connected to each other while thesource signal line and the source signal line driver circuit 915 areconnected to each other. A signal is selectively input in the signalline in the y^(th) row from the writing gate signal line driver circuit913 and the first transistor is turned on while signals for writing areinput in the source signal lines in the first to last columns from thesource signal line driver circuit 915. By inputting these signals, thelight emitting element in the y^(th) row emits light or no light.

After terminating the writing period in the y^(th) row as mentionedabove, the erasing period immediately starts in the x+1^(th) row.Therefore, the gate signal line and the writing gate signal line drivercircuit 913 are disconnected to each other and the source signal line isconnected to the power source 916 by turning the switch 918 on/off.Also, the gate signal line and the writing gate signal line drivercircuit 913 are disconnected to each other and the gate signal line isconnected to the erasing gate signal line driver circuit 914. A signalis selectively input in the gate signal line in the x+1^(th) row fromthe erasing gate signal line driver circuit 914 to input the signal inthe first transistor while an erasing signal is input therein from thepower source 916. Upon terminating the erasing period in the x+1^(th)row in this manner, the writing period immediately starts in the y^(th)row. The erasing period and the writing period may be repeatedalternatively until the erasing period of the last row.

Although the writing period of the y^(th) row is provided between theerasing period of the x^(th) row and the erasing period of the x+1^(th)row in this embodiment mode, the present invention is not limitedthereto. The writing period of the y^(th) row may be provided betweenthe erasing period in the x−1^(th) row and the erasing period in thex^(th) row.

In this embodiment mode, when the non-light emitting period 504 d isprovided like the sub-frame 504, the operation of disconnecting theerasing gate signal line driver circuit 914 from one gate signal lineand connecting the writing gate signal line driver circuit 913 toanother gate signal line is carried out repeatedly. This operation maybe performed in a frame in which a non-light emitting period is notparticularly provided.

[Embodiment Mode 4]

An example of a cross sectional view of a light emitting deviceincluding a light emitting element of the invention will be describedwith reference to FIGS. 7A to 7C.

In each of FIGS. 7A to 7C, a region surrounded by a dashed linerepresents a transistor 11 that is provided for driving a light emittingelement 12 of the invention. The light emitting element 12 of theinvention comprises a layer 15 between a first electrode 13 and a secondelectrode 14. A drain of the transistor 11 and the first electrode 13are electrically connected to each other by a wiring 17 passing througha first interlayer insulating film 16 (16 a, 16 b and 16 c). The lightemitting element 12 is isolated from another light emitting elementsprovided adjacent to the light emitting element 12 by a partition walllayer 18. The light emitting device of the invention having thisstructure is provided over a substrate 10 in this embodiment mode.

The transistor 11 as shown in each FIGS. 7A to 7C is a top-gate typetransistor in which a gate electrode is provided on a side of asemiconductor layer opposite to the substrate. Further the structure ofthe transistor 11 is not particularly limited. For example, abottom-gate type transistor may be employed. In the case of using abottom-gate type transistor, either a transistor in which a protectionfilm is formed on a semiconductor layer of a channel (a channelprotection type transistor) or a transistor in which a part of asemiconductor layer of a channel is etched (a channel etched typetransistor) may be used.

The semiconductor layer included in the transistor 11 may be any of acrystalline semiconductor, an amorphous semiconductor, a semiamorphoussemiconductor, and the like.

Concretely, a semiamorphous semiconductor has an intermediate structurebetween an amorphous structure and a crystalline structure (including asingle crystalline structure and a polycrystalline structure), and athird condition that is stable in term of free energy. The semiamorphoussemiconductor further includes a crystalline region having a short rangeorder along with lattice distortion. A crystal grain with a size of 0.5to 20 nm is included in at least a part of an semiamorphoussemiconductor film. Raman spectrum is shifted toward lower wavenumbersthan 520 cm⁻¹. The diffraction peaks of (111) and (220), which arebelieved to be derived from silicon crystal lattice, are obseived in thesemiamorphous semiconductor by the X-ray diffraction. The semiamorphoussemiconductor contains hydrogen or halogen of at least 1 atom % or morefor terminating dangling bonds. The semiamorphous semiconductor is alsoreferred to as a microcrystalline semiconductor. The semiamorphoussemiconductor is formed by glow discharge decomposition with silicidegas (plasma CVD). As for the silicide gas, SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃,SiCl₄, SiF₄ and the like can be used. The silicide gas may also bediluted with H₂, or a mixture of H₂ and one or more of rare gas elementsselected from He, Ar, Kr and Ne. The dilution ratio is set to be in therange of 1:2 to 1:1,000. The pressure is set to be approximately in therange of 0.1 to 133 Pa. The power frequency is set to be 1 to 120 MHz,preferably, 13 to 60 MHz. The substrate heating temperature may be setto be 300° C. or less, more preferably, 100 to 250° C. With respect toimpurity elements contained in the film, each concentration ofimpurities for atmospheric constituents such as oxygen, nitrogen andcarbon is preferably set to be 1×10²⁰/cm³ or less. In particular, theoxygen concentration is set to be 5×10¹⁹/cm³ or less, preferably,1×10¹⁹/cm³ or less. Further, the mobility of a TFT (thin filmtransistor) using an amorphous semiconductor is set to be about 1 to 10m²/Vsec.

As a specific example of a crystalline semiconductor layer, asemiconductor layer made from single crystal silicon, polycrystallinesilicon, silicon germanium, or the like can be cited. These materialsmay be formed by laser crystallization. For example, these materials maybe formed by crystallization with use of the solid phase growth methodusing nickel and the like.

When a semiconductor layer is made from an amorphous substance, e.g.,amorphous silicon, it is preferable to use a light emitting device withcircuits including only n-channel transistors as the transistor 11 andanother transistor (a transistor included in a circuit for driving alight emitting element). Alternatively, a light emitting device withcircuits including either n-channel transistors or p-channel transistorsmay be employed. Also, a light emitting device with circuits includingboth an n-channel transistor and a p-channel transistor may be used.

The first interlayer insulating film 16 may include plural layers (e.g.,first interlayer insulating films 16 a, 16 b and 16 c) as shown in FIGS.7A and 7C or a single layer. The interlayer insulating film 16 a is madefrom an inorganic material such as silicon oxide and silicon nitride.The interlayer insulating film 16 b is made from acrylic, siloxane(which is a substance that has a skeleton structure formed by silicon(Si)-oxygen (O) bonds and includes at least hydrogen as itssubstituent), or a substance with a self-planarizing property that canbe formed by applying a liquid such as silicon oxide. The interlayerinsulating film 16 c is made from a silicon nitride film containingargon (Ar). The substances constituting the respective layers are notparticularly limited thereto. Therefore, substances other than theabove-mentioned substances may be employed. Alternatively, theabove-mentioned substances may be used in combination with the substanceother than the above-mentioned substances. Accordingly, the firstinterlayer insulating film 16 may be formed by using both an inorganicmaterial and an organic material or by using any one of inorganic andorganic materials.

The edge portion of the partition wall layer 18 preferably has a shapein which the radius of curvature is continuously varied. This partitionwall layer 18 is formed by using acrylic, siloxane, resist, siliconoxide, and the like. Further, the partition wall layer 18 may be madefrom any one of or both an inorganic film and an organic film.

FIGS. 7A and 7C show the structures in which only the first interlayerinsulating films 16 are sandwiched between the transistors 11 and thelight emitting elements 12. Alternatively, as shown in FIG. 7B, thefirst interlayer insulating film 16 (16 a and 16 b) and a secondinterlayer insulting film 19 (19 a and 19 b) may be provided between thetransistor 11 and the light emitting element 12. In the light emittingdevice as shown in FIG. 7B, the first electrode 13 passes through thesecond interlayer insulating film 19 to be electrically connected to thewiring 17.

The second interlayer insulating film 19 may include either plurallayers or a single layer as well as the first interlayer insulating film16. The interlayer insulating film 19 a is made from acrylic, siloxane(which is a substance that has a skeleton structure formed by silicon(Si)-oxygen (O) bonds and includes at least hydrogen as itssubstituent), or a substance with a self-planarizing property that canbe formed by applying a liquid such as silicon oxide. The interlayerinsulating film 19 b is made from a silicon nitride film containingargon (Ar). The substances constituting the respective second interlayerinsulating layers are not particularly limited thereto. Therefore,substances other than the above-mentioned substances may be employed.Alternatively, the above-mentioned substances may be used in combinationwith a substance other than the above-mentioned substances. Accordingly,the second interlayer insulating film 19 may be formed by using both aninorganic material and an organic material or by using any one ofinorganic and organic materials.

When the first electrode and the second electrode are both formed byusing a substance with a light transmitting property in the lightemitting element 12, light generated in the light emitting element canbe emitted through both the first electrode 13 and the second electrode14 as shown in arrows in FIG. 7A. When only the second electrode 14 ismade from a substance with a light transmitting property, lightgenerated in the light emitting element 12 can be emitted only throughthe second electrode 14 as shown in an arrow of FIG. 7B. In this case,the first electrode 13 is preferably made from a material with highreflectance or a film (reflection film) made from a material with highreflectance is preferably provided under the first electrode 13. Whenonly the first electrode 13 is made from a substance with a lighttransmitting property, light generated in the light emitting element 12can be emitted only through the first electrode 13 as shown in an arrowof FIG. 7C. In this case, the second electrode 14 is preferably madefrom a material with high reflectance or a reflection film is preferablyprovided over the second electrode 14.

Moreover, the light emitting element 12 may has a structure in which thefirst electrode 13 servers as an anode and the second electrode 14servers as a cathode or a structure in which the first electrode 13serves as a cathode and the second electrode 14 serves as an anode. Inthe former case, the transistor 11 is a p-channel transistor. In thelatter case, the transistor 11 is an n-channel transistor.

[Embodiment Mode 5]

Since a light emitting device according to the present invention has anexcellent moisture resistant property, an electronic appliance capableof displaying images preferably for a long time or an electronicappliance capable of lighting preferably for a long time can be obtainedby using the light emitting device of the invention.

Examples of electronic appliances mounted with the light emittingdevices of the invention are illustrated in FIGS. 8A to 8C.

FIG. 8A is a laptop personal computer manufactured according to theinvention, including a main body 5521, a housing 5522, a display portion5523, a keyboard 5524 and the like. The laptop personal computer can beachieved by incorporating the light emitting device including the lightemitting element of the invention thereinto.

FIG. 8B is a cellular phone manufactured according to the invention,including a main body 5552, a display portion 5551, an audio outputportion 5554, an audio input portion 5555, operation switches 5556 and5557, an antenna 5553 and the like. The cellular phone can be achievedby incorporating the light emitting device including the light emittingelement of the invention thereinto.

FIG. 8C is a television set manufactured according to the invention,including a display portion 5531, a housing 5532, speakers 5533 and thelike. The television set can be achieved by incorporating the lightemitting device including the light emitting element of the inventionthereinto.

As set forth above, the light emitting devices of the invention aresuitable to be used as the display portions of various kinds ofelectronic appliances.

Further the light emitting devices having the light emitting elements ofthe invention are mounted on the laptop personal computer, the cellularphone and the television set. However, the light emitting devices havingthe light emitting elements of the invention can be mounted on apersonal computer, a car navigation system, a lighting appliance and thelike.

[Embodiment 1]

An embodiment of the invention will be described with reference to FIG.9.

Indium tin oxide was formed to have a thickness of 110 nm over a glasssubstrate 301 by sputtering so as to form a layer 301 containing indiumtin oxide.

A layer 302 containing α-NPD and molybdenum oxide with a thickness of 50nm was formed over the layer 301 containing indium tin oxide byco-evaporation of α-NPD and molybdenumnn oxide such that the weightratio of α-NPD to molybdenum oxide satisfied 1:0.25. Further, theco-evaporation indicates an evaporation method in which evaporation issimultaneously performed from a plurality of evaporation sources.

Next, α-NPD was formed over the layer 302 containing α-NPD andmolybdenum oxide by evaporation to form a layer 303 containing α-NPDwith a thickness of 10 nm.

A layer 304 containing Alq₃, rubrene and DCJTI with a thickness of 37.5nm was formed over the layer 303 containing α-NPD by co-evaporation ofAlq₃, rubrene and DCJTI such that the Alq₃-rubrene-DCJTI weight ratiosatisfied 1:1:0.02.

Then, Alq₃ was formed on the layer 304 containing Alq₃, rubrene andDCJTI by evaporation to form a layer 305 containing Alq₃ with athickness of 27.5 nm.

A layer 306 containing BCP and lithium with a thickness of 10 nm wasformed over the layer 305 containing Alq₃ by co-evaporation of BCP andlithium (Li) such that the weight ratio of BCP to lithium satisfied1:0.005.

A layer 307 containing α-NPD and molybdenum oxide with a thickness of 50nm was formed over the layer 306 containing BCP and lithium byco-evaporation of α-NPD and molybdenum oxide such that the weight ratioof α-NPD to molybdenum oxide satisfied 1:0.25.

Next, α-NPD was formed over the layer 307 containing α-NPD andmolybdenum oxide by evaporation to form a layer 308 containing α-NPDwith a thickness of 10 nm.

A layer 309 containing Alq₃ and coumarin 6 with a thickness of 37.5 nmwas formed over the layer 308 containing α-NPD by co-evaporation of Alq₃and coumarin 6 such that the weight ratio of Alq₃ to coumarin 6satisfied 1:0.005.

Next, Alq₃ was formed over the layer 309 containing Alq₃ and coumarin 6by evaporation to form a layer 310 containing Alq₃ with a thickness of27.5 nm.

A layer 311 containing BCP and lithium with a thickness of 10 nm wasformed over the layer 310 containing Alq₃ by co-evaporation of BCP andlithium (Li) such that the weight ratio of BCP to lithium satisfied1:0.005.

A layer 312 containing α-NPD and molybdenum oxide with a thickness of 50nm was formed over the layer 311 containing BCP and lithium byco-evaporation of α-NPD and molybdenum oxide such that the weight ratioof α-NPD to molybdenum oxide satisfied 1:0.25.

Subsequently, α-NPD was formed over the layer 312 containing α-NPD andmolybdenum oxide by evaporation to form a layer 313 containing α-NPDwith a thickness of 10 nm.

Next, t-BuDNA was formed over the layer 313 containing α-NPD byevaporation to form a layer 314 containing t-BuDNA with a thickness of37.5 nm.

Alq₃ was next formed over the layer 314 containing t-BuDNA byevaporation to form a layer 315 containing Alq₃ with a thickness of 27.5nm.

A layer 316 containing BCP and lithium with a thickness of 10 nm wasformed over the layer 315 containing Alq₃ by co-evaporation of BCP andlithium (Li) such that the weight ratio of BCP to lithium (Li) satisfied1:0.005.

Subsequently, aluminum was formed over the layer 316 containing BCP andlithium by evaporation to form a layer 317 containing aluminum with athickness of 200 nm.

In the thus-manufactured light emitting element, the layer 301containing indium tin oxide serves as an anode and the layer 317containing aluminum serves as a cathode.

The layer 302 containing α-NPD and molybdenum oxide has a property ofinjecting holes into the layer 303 containing α-NPD. Also, the layer 307containing α-NPD and molybdenum oxide has a property of injecting holesinto the layer 308 containing α-NPD. The layer 312 containing α-NPD andmolybdenum oxide has a property of injecting holes into the layer 313containing α-NPD.

The layer 303 containing α-NPD has a property of transporting theinjected holes to the layer 304 containing Alq₃, rubrene and DCJTI. Thelayer 308 containing α-NPD has a property of transporting the injectedholes to the layer 309 containing Alq₃ and coumarin 6. The layer 313containing α-NPD serves as a hole transporting layer for transportingthe injected holes to the layer 314 containing t-BuDNA.

The layer 306 containing BCP and lithium has a property of injectingelectrons in the layer 305 containing Alq₃. Further, the layer 311containing BCP and lithium has a property of injecting electrons to thelayer 310 containing Alq₃. The layer 316 containing BCP and lithium hasa property of injecting electrons into the layer 315 containing Alq₃.

The layer 305 containing Alq₃ has a property of transporting theinjected electrons to the layer 304 containing Alq₃, rubrene and DCJTI.The layer 310 containing Alq₃ has a property of transporting electronsinjected from the layer 311 containing BCP and lithium to the layer 309containing Alq₃ and coumarin 6. The layer 315 containing Alq₃ serves asan electron transporting layer that transports electrons injected fromthe layer 316 containing BCP and lithium to the layer 314 containingt-BuDNA.

In the layers 302, 307 and 312 containing α-NPD and molybdenum oxide,molybdenum oxide serves as an electron acceptor. Further, in the layers306, 311 and 316 containing BCP and lithium, lithium serves as anelectron donor.

In this light emitting element, when applying a voltage to the layer 301containing indium tin oxide and the layer 317 containing aluminum,current flows through the layer 301 containing indium tin oxide and thelayer 317 containing aluminum. Therefore, the layer 304 containing Alq₃,rubrene and DCJTI emits light with a peak in a wavelength range of 600to 680 nm. The layer 309 containing Alq³ and coumarin 6 emits light witha peak in a wavelength range of 500 to 550 nm. The layer 314 containingt-BuDNA emits light with a peak in a wavelength range of 420 to 480 nm.Light generated in these layers is emitted to the outside through thelayer 301 containing indium tin oxide. As can be seen from the abovedescription, in the light emitting element of this embodiment, the layerexhibiting light with a shorter wavelength of 420 to 480 nm is providedto be closer to a layer with high reflectance such as the layer 315containing aluminum than the layer exhibiting light with a longerwavelength of 600 to 680 nm. Consequently, interference of lightgenerated in the layers and light reflected by the layer 317 containingaluminum can be reduced.

The emission spectrums in the case where the light emitting elementmanufactured in this embodiment emits light will be shown in FIG. 10. InFIG. 10, a horizontal axis indicates a wavelength (nm) and alongitudinal axis indicates a emission intensity (an arbitrary unit).According to FIG. 10, it is known that the light emitting elementmanufactured in this embodiment emits light at a wavelength of 450 to620 nm. The CIE chromaticity coordinate at 0.979 mA are x=033, y=0.46.Therefore, it is known that the light emitting element manufactured inthis embodiment emits white light.

Since the light emitting element of this embodiment is manufactured byusing a substance with a low moisture absorbing property such asmolybdenum oxide, the light emitting element is hardly deteriorated bymoisture intruding into the light emitting element. In addition, thelight emitting element of this embodiment can emit white light.Moreover, interference of light emitted from the light emitting elementand reflected light is hardly caused in the light emitting element ofthis embodiment, and hence, color tone of light emitted form the lightemitting element can be controlled easily.

[Embodiment 2]

The present embodiment will show experimental results obtained byexamining whether or not molybdenum oxide serves as a substance with anelectron accepting property with respect to α-NPD.

In this experiment, three kinds of thin films, i.e., a thin film Ahaving the same structure as the layer 302 containing α-NPD andmolybdenum oxide, a thin film B containing molybdenum oxide and a thinfilm C containing α-NPD were formed on glass substrates respectively byvacuum evaporation. The transmission spectrums of respective thin filmswere compared.

The experimental results are shown in FIG. 11. A horizontal axisrepresents the wavelength while a perpendicular axis represents thetransmittance. With respect to the thin film A having the same structureas the layer 302 (described in Embodiment 1) containing α-NPD andmolybdenum oxide, a broad peak, which cannot be observed in the thinfilm B containing molybdenum oxide and the thin film C containing α-NPD,can be observed in the vicinity of 500 nm (a region surrounded by adashed line in the drawing) as shown in FIG. 11. It is thought that thisis an energy level that is newly generated due to electron transfercaused by transferring electrons to molybdenum oxide from α-NPD. As aconsequence, it is known that molybdenum oxide exhibits an electronaccepting property with respect to α-NPD.

The present application is based on Japanese Priority Application No.2004-152491 filed on May 21, 2004 with the Japan Patent Office, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting device comprising; ananode; a first layer over the anode, the first layer comprising a firstorganic compound; a second layer over the first layer, the second layercomprising molybdenum oxide; a third layer over the second layer, thethird layer comprising a second organic compound; and a cathode over thethird layer.
 2. The light-emitting device according to claim 1, whereinthe first layer and the second layer are configured to emit light when avoltage is applied between the anode and the cathode.
 3. Thelight-emitting device according to claim 1, wherein the first layercomprises a plurality of organic compound layers, and wherein the firstorganic compound is included in one of the plurality of organic compoundlayers.
 4. The light-emitting device according to claim 1, wherein thethird layer comprises a plurality of organic compound layers, andwherein the second organic compound is included in one of the pluralityof organic compound layers.
 5. The light-emitting device according toclaim 1, wherein the first organic compound is a light-emittingmaterial.
 6. The light-emitting device according to claim 1, wherein thesecond organic compound is a light-emitting material.
 7. Thelight-emitting device according to claim 1, further comprising a fourthlayer between the first layer and the second layer, wherein the fourthlayer is in contact with the second layer, and wherein the fourth layercomprises an electron-transporting material and a material selected froman alkali metal and an alkali earth metal.
 8. The light-emitting deviceaccording to claim 2, wherein an emission color of the first layer isdifferent from an emission color of the third layer.
 9. Thelight-emitting device according to claim 8, wherein the emission colorof the first layer is complementary to the emission color of the thirdlayer.
 10. The light-emitting device according to claim 3, wherein theplurality of organic compound layers include a layer comprising ahole-transporting material and an electron acceptor with respect to thehole-transporting material, and wherein the layer is in contact with theanode.
 11. The light-emitting device according to claim 10, wherein theelectron acceptor is molybdenum oxide.
 12. An electronic appliancecomprising the light-emitting device according to claim
 1. 13. Alighting appliance comprising the light-emitting device according toclaim
 1. 14. An electronic appliance comprising a light-emitting device,the light-emitting device comprising: a display portion comprising aplurality of pixels arranged in a matrix form, each of the plurality ofpixels comprising a transistor and a light-emitting element; a drivercircuit electrically connected to the display portion; an external inputterminal electrically connected to the driver circuit through a wiring;and a printed wiring board attached to the external input terminal,wherein the electronic appliance further comprises a main body to whichan audio input portion, an audio output portion, an operation key, andthe display portion are incorporated, and wherein the light-emittingelement comprises: an anode; a first layer over the anode, the firstlayer comprising a first organic compound; a second layer over and incontact with the first layer, the second layer comprising molybdenumoxide; a third layer over and in contact with the second layer, thethird layer comprising a second organic compound; and a cathode over thethird layer.
 15. The electronic appliance according to claim 14, whereinthe electronic appliance is a mobile phone.
 16. An electronic appliancecomprising a light-emitting device, the light-emitting devicecomprising: a display portion comprising a plurality of pixels arrangedin a matrix form, each of the plurality of pixels comprising atransistor and a light-emitting element; a driver circuit electricallyconnected to the display portion; an external input terminalelectrically connected to the driver circuit through a wiring; and aprinted wiring board attached to the external input terminal, whereinthe electronic appliance further comprises a main body to which thedisplay portion is incorporated, and wherein the light-emitting elementcomprises: an anode; a first layer over the anode, the first layercomprising a first organic compound; a second layer over and in contactwith the first layer, the second layer comprising molybdenum oxide; athird layer over and in contact with the second layer, the third layercomprising a second organic compound; and a cathode over the thirdlayer.
 17. The electronic appliance according to claim 16, wherein theelectronic appliance is selected from a computer and a television set.18. The light-emitting device according to claim 1, wherein the secondlayer further comprises an organic compound which is mixed with themolybdenum oxide in the second layer.
 19. The light-emitting deviceaccording to claim 1, wherein the second layer is spaced from both theanode and the cathode.
 20. The light-emitting device according to claim18, wherein the organic compound is an aromatic amine.